Optical waveguide and optical concentration measuring apparatus comprising a support with a shifted connecting portion

ABSTRACT

An optical waveguide ( 10 ) includes a substrate ( 15 ), a core layer ( 11 ) that extends along the longitudinal direction and through which infrared light IR can propagate, and a support ( 17 ) formed from a material with a smaller refractive index than the core layer ( 11 ) and configured to connect at least a portion of the substrate ( 15 ) and at least a portion of the core layer ( 11 ) to support the core layer with respect to the substrate ( 15 ). A connecting portion ( 171 ) of the support ( 17 ) connected to the core layer ( 11 ) is shifted from the position having the shortest distance from the center to the outer surface in a cross-section perpendicular to the longitudinal direction of the core layer ( 11 ).

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of JapanesePatent Application No. 2017-041799 filed Mar. 6, 2017, Japanese PatentApplication No. 2017-063385 filed Mar. 28, 2017, and Japanese PatentApplication No. 2017-229493 filed Nov. 29, 2017, the entire contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an optical waveguide and an opticalconcentration measuring apparatus.

BACKGROUND

When the refractive index of a material forming a structure, such as athin film formed by crystals or the like, is greater than the refractiveindex of material outside the structure, light propagating through thestructure progresses while repeatedly undergoing total internalreflection at the boundary between the structure and the outside of thestructure.

As illustrated in FIG. 35, when light L propagating through a structure51 undergoes total internal reflection at the boundary between thestructure 51 and a substance 53, the light L not only propagates insidethe structure 51 but also extends into the substance 53 that has a smallrefractive index. This extension is referred to as an evanescent wave,which may be absorbed by the substance adjacent to the structure 51while the light L propagates through the structure 51. In FIG. 35, theintensity of the light L propagating inside the structure 51 isindicated as light intensity E1, whereas the intensity of the evanescentwave is indicated as light intensity E2. Therefore, the substance 53 incontact with the structure 51 can be detected, identified, or the likefrom the change in intensity of the light L propagating through thestructure 51. An analytical method using the above-described principleof evanescent waves is referred to as the attenuated total reflection(ATR) method and is used in the chemical composition analysis ofsubstances, for example.

Patent literature (PTL) 1 proposes an optical waveguide sensor in whichthe ATR method is applied to a sensor. This optical waveguide sensor hasa core layer formed on a substrate, allows light to pass through thecore layer, and uses an evanescent wave to detect a substance in contactwith the core layer.

The sensor sensitivity of a sensor using the ATR method can be improvedby increasing the amount of interaction between the evanescent wave andthe substance to be measured and by reducing the absorption of light bymaterial other than the substance to be measured. Therefore, a pedestalstructure (see FIG. 36), in which the layer below the core layer isminimized to leave a portion of the core layer floating, has beenproposed in recent years, as in non-patent literature (NPL) 1.

Infrared light is typically used as the light propagated through thecore layer. Substances have the property of selectively absorbinginfrared light of particular wavelengths. A substance can therefore beanalyzed or sensed by propagating infrared light in accordance with theabsorption spectrum of the substance to be measured.

CITATION LIST Patent Literature

PTL 1: JP2005-300212A

Non-patent Literature

NPL 1: Pao Tai Lin et al., “Si-CMOS compatible materials and devices formid-IR microphotonics”, Optical Materials Express, Vol. 3, Issue 9, pp.1474-1487 (2013)

SUMMARY Technical Problem

A sensor for detecting a substance to be measured, such as a gas or aliquid, is required to be capable of stably detecting the substance tobe measured, in a variety of modes of use, with high sensitivity.

It is an objective of the present disclosure to provide an opticalwaveguide and an optical concentration measuring apparatus that canstably detect a substance to be measured, in a variety of modes of use,with high sensitivity.

Solution to Problem

To achieve the aforementioned objective, an optical waveguide accordingto an embodiment of the present disclosure is an optical waveguide foruse in an optical concentration measuring apparatus for measuringconcentration of a gas to be measured or a liquid to be measured, theoptical waveguide including a substrate, a core layer that extends alonga longitudinal direction and through which light can propagate, and asupport formed from a material with a smaller refractive index than thecore layer and configured to connect at least a portion of the substrateand at least a portion of the core layer to support the core layer withrespect to the substrate. A connecting portion of the support connectedto the core layer is shifted from a position having a shortest distancefrom a center to an outer surface in a cross-section perpendicular tothe longitudinal direction of the core layer.

To achieve the aforementioned objective, an optical waveguide accordingto another embodiment of the present disclosure includes a substrate, acore layer that extends along a longitudinal direction and through whichlight can propagate, and a support formed from a material with a smallerrefractive index than the core layer and configured to connect at leasta portion of the substrate and at least a portion of the core layer tosupport the core layer with respect to the substrate. At least a portionof the core layer is provided in a manner allowing contact with a gas ora liquid. A connecting portion of the support connected to the corelayer is shifted from a position having a shortest distance from acenter to an outer surface in a cross-section perpendicular to thelongitudinal direction of the core layer.

To achieve the aforementioned objective, an optical concentrationmeasuring apparatus according to an embodiment of the present disclosureincludes the optical waveguide according to any embodiment of thepresent disclosure, a light source capable of causing light to enter thecore layer, and a detector capable of detecting light that haspropagated through the core layer.

Advantageous Effect

The present disclosure can provide an optical waveguide and an opticalconcentration measuring apparatus that can stably detect a substance tobe measured, in a variety of modes of use, with high sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates the schematic configuration of an optical waveguide10 and an optical concentration measuring apparatus 1 according to afirst embodiment of the present disclosure and illustrates sensing bythe ATR method using the optical concentration measuring apparatus 1;

FIG. 2 is a cross-sectional end view of the optical waveguide 10 and theoptical concentration measuring apparatus 1, along the A-A line in FIG.1, illustrating the schematic configuration of the optical waveguide 10and the optical concentration measuring apparatus 1 according to thefirst embodiment of the present disclosure;

FIG. 3A is a manufacturing process plan view (part 1) of the opticalwaveguide 10 to illustrate a method of manufacturing the opticalwaveguide 10 according to the first embodiment of the presentdisclosure;

FIG. 3B is a cross-sectional manufacturing process end view (part 1) ofthe optical waveguide 10, along the B-B line in FIG. 3A, to illustratethe method of manufacturing the optical waveguide 10 according to thefirst embodiment of the present disclosure;

FIG. 4A is a manufacturing process plan view (part 2) of the opticalwaveguide 10 to illustrate the method of manufacturing the opticalwaveguide 10 according to the first embodiment of the presentdisclosure;

FIG. 4B is a cross-sectional manufacturing process end view (part 2) ofthe optical waveguide 10, along the B-B line in FIG. 4A, to illustratethe method of manufacturing the optical waveguide 10 according to thefirst embodiment of the present disclosure;

FIG. 5A is a manufacturing process plan view (part 3) of the opticalwaveguide 10 to illustrate the method of manufacturing the opticalwaveguide 10 according to the first embodiment of the presentdisclosure;

FIG. 5B is a cross-sectional manufacturing process end view (part 3) ofthe optical waveguide 10, along the B-B line in FIG. 5A, to illustratethe method of manufacturing the optical waveguide 10 according to thefirst embodiment of the present disclosure;

FIG. 6A is a manufacturing process plan view (part 4) of the opticalwaveguide 10 to illustrate the method of manufacturing the opticalwaveguide 10 according to the first embodiment of the presentdisclosure;

FIG. 6B is a cross-sectional manufacturing process end view (part 4) ofthe optical waveguide 10, along the B-B line in FIG. 6A, to illustratethe method of manufacturing the optical waveguide 10 according to thefirst embodiment of the present disclosure;

FIG. 7 illustrates an optical waveguide 60 according to a secondembodiment of the present disclosure;

FIG. 8 illustrates an optical waveguide 70 according to a thirdembodiment of the present disclosure, illustrating multi-modepropagation of light through a core layer 11;

FIG. 9A is a manufacturing process plan view (part 1) of the opticalwaveguide 10 to illustrate the method of manufacturing the opticalwaveguide 70 according to the third embodiment of the presentdisclosure;

FIG. 9B is a cross-sectional manufacturing process end view (part 1) ofthe optical waveguide 70, along the C-C line in FIG. 9A, to illustratethe method of manufacturing the optical waveguide 70 according to thethird embodiment of the present disclosure;

FIG. 10A is a manufacturing process plan view (part 2) of the opticalwaveguide 70 to illustrate the method of manufacturing the opticalwaveguide 70 according to the third embodiment of the presentdisclosure;

FIG. 10B is a cross-sectional manufacturing process end view (part 2) ofthe optical waveguide 70, along the C-C line in FIG. 10A, to illustratethe method of manufacturing the optical waveguide 70 according to thethird embodiment of the present disclosure;

FIG. 11 illustrates the schematic configuration of an optical waveguide80 and an optical concentration measuring apparatus 8 according to afourth embodiment of the present disclosure and illustrates sensing bythe ATR method using the optical concentration measuring apparatus 8;

FIG. 12 is a cross-sectional end view of the optical waveguide 80, alongthe D-D line and F-F line in FIG. 11, illustrating the schematicconfiguration of the optical waveguide 80 according to the fourthembodiment of the present disclosure;

FIG. 13 is a cross-sectional end view of the optical waveguide 80, alongthe E-E line in FIG. 11, illustrating the schematic configuration of theoptical waveguide 80 according to the fourth embodiment of the presentdisclosure;

FIG. 14 is a plan view of the optical waveguide 80, as seen from a lightsource 20 or light detector 40 side, to illustrate the arrangement of afirst support 87 x and a second support 87 y of FIG. 11;

FIG. 15 is a plan view of an SOI substrate 100 to illustrate a method ofmanufacturing the optical waveguide 80 according to the fourthembodiment of the present disclosure;

FIG. 16 is a cross-sectional end view of the SOI substrate 100 of FIG.15 along the G-G line, I-I line, H-H line, and J-J line;

FIG. 17 is a plan view of an optical waveguide main portion 80 a toillustrate the method of manufacturing the optical waveguide 80according to the fourth embodiment of the present disclosure;

FIG. 18 is a cross-sectional end view of the optical waveguide mainportion 80 a of FIG. 17 along the G-G line, I-I line, H-H line, and J-Jline;

FIG. 19 is a plan view of the optical waveguide main portion 80 apartially covered by a mask layer to illustrate the method ofmanufacturing the optical waveguide 80 according to the fourthembodiment of the present disclosure;

FIG. 20 is a cross-sectional end view of the optical waveguide mainportion 80 a of FIG. 19 along the G-G line and I-I line;

FIG. 21 is a cross-sectional end view of the optical waveguide mainportion 80 a of FIG. 19 along the H-H line;

FIG. 22 is a cross-sectional end view of the optical waveguide mainportion 80 a of FIG. 19 along the J-J line;

FIG. 23 is a plan view of the optical waveguide main portion 80 a, witha portion of a BOX layer 17 a removed, to illustrate the method ofmanufacturing the optical waveguide 80 according to the fourthembodiment of the present disclosure;

FIG. 24 is a cross-sectional end view of the optical waveguide mainportion 80 a of FIG. 23 along the G-G line and I-I line;

FIG. 25 is a cross-sectional end view of the optical waveguide mainportion 80 a of FIG. 23 along the H-H line;

FIG. 26 is a cross-sectional end view of the optical waveguide mainportion 80 a of FIG. 23 along the J-J line;

FIG. 27 is an end view illustrating the schematic configuration of anoptical waveguide 80 according to a modification to the fourthembodiment of the present disclosure;

FIG. 28 illustrates a schematic configuration of an optical waveguide 90and an optical concentration measuring apparatus 9 according to a fifthembodiment of the present disclosure and illustrates sensing by the ATRmethod using the optical concentration measuring apparatus 9;

FIG. 29 is a cross-sectional end view of the optical waveguide 90 andthe optical concentration measuring apparatus 9, along the K-K line inFIG. 28, illustrating the schematic configuration of the opticalwaveguide 90 and the optical concentration measuring apparatus 9according to the fifth embodiment of the present disclosure;

FIG. 30 is a manufacturing process end view (part 1) illustrating amethod of manufacturing the optical waveguide 90 and the opticalconcentration measuring apparatus 9 according to the fifth embodiment ofthe present disclosure;

FIG. 31 is a manufacturing process end view (part 2) illustrating themethod of manufacturing the optical waveguide 90 and the opticalconcentration measuring apparatus 9 according to the fifth embodiment ofthe present disclosure;

FIG. 32 illustrates the optical waveguide 90 and the opticalconcentration measuring apparatus 9 according to the fifth embodiment ofthe present disclosure, illustrating a nitrogen concentrationdistribution when silicon is oxidized under an atmosphere including NO;

FIG. 33 illustrates another schematic configuration of the opticalwaveguide 90 and the optical concentration measuring apparatus 9according to the fifth embodiment of the present disclosure andillustrates sensing by the ATR method using the optical concentrationmeasuring apparatus 9;

FIG. 34 is a cross-sectional end view of the optical waveguide 90 andthe optical concentration measuring apparatus 9, along the L-L line inFIG. 33, illustrating another schematic configuration of the opticalwaveguide 90 and the optical concentration measuring apparatus 9according to the fifth embodiment of the present disclosure;

FIG. 35 illustrates an evanescent wave of light propagating through anoptical waveguide; and

FIG. 36 is a cross-sectional view of single-mode propagation of light Lthrough a core layer 17′ of an optical waveguide 10′ that has aconventional pedestal structure.

DETAILED DESCRIPTION

Embodiments of the present disclosure are now described, but thefollowing embodiments do not limit the claimed subject matter.Furthermore, not all combinations of features described in theembodiments are necessarily essential to the solution to the problem ofthe present disclosure.

<Optical Waveguide>

An optical waveguide according to a first embodiment of the presentdisclosure is an optical waveguide for use in an optical concentrationmeasuring apparatus for measuring concentration of a gas to be measuredor a liquid to be measured. The optical waveguide includes a substrate,a core layer that extends along a longitudinal direction and throughwhich light can propagate, and a support formed from a material with asmaller refractive index than the core layer and configured to connectat least a portion of the substrate and at least a portion of the corelayer to support the core layer with respect to the substrate. Aconnecting portion of the support connected to the core layer is shiftedfrom a position having a shortest distance from a center to an outersurface in a cross-section perpendicular to the longitudinal directionof the core layer. The longitudinal direction is the direction ofgreatest extension in a three-dimensional structure that is elongated inat least one direction. The longitudinal direction may refer not only toa linear direction but also to a curved direction. The cross-sectionperpendicular to the longitudinal direction of the core layer may be,but is not limited to, a rectangle. The cross-section may have any shapethat is not circular and for which the distance from the center of thecross-section to the outer surface varies with rotation about an axis atthe center of the cross-section. The refractive index refers to therefractive index relative to light of any wavelength or light of aparticular wavelength. Particularly in an optical concentrationmeasuring apparatus, the light of a particular wavelength is the lightpropagating through the core layer. The width direction is a directionperpendicular to the longitudinal direction of the core layer andparallel to the principal surface of the substrate in the presentembodiment. The principal surface of the substrate is a surfaceperpendicular to the thickness direction of the substrate and refers, inthe present embodiment, to the surface with the greatest area among thesix surfaces forming the substrate. At least a portion of the core layermay be provided in a manner allowing contact, via a film that is thinnerthan the wavelength of light propagating through the core layer, withthe gas to be measured or the liquid to be measured.

The connecting portion of the support connected to the core layer in theoptical waveguide according to the first embodiment is shifted from theposition having the shortest distance from the center to the outersurface in a cross-section perpendicular to the longitudinal directionof the core layer. In other words, in a plane orthogonal to thelongitudinal direction that becomes the direction along which lightpropagates, the connecting portion is provided at a position fartheraway than the position closest to the center through which light mainlypropagates. Consequently, the optical waveguide according to the firstembodiment can expand the region of interaction between an evanescentwave and the gas or liquid to be measured while decreasing absorption ofthe evanescent wave by the support. Therefore, the measurementsensitivity of an optical concentration measuring apparatus includingthe optical waveguide according to the first embodiment can beincreased.

An optical waveguide according to a second embodiment of the presentdisclosure includes a substrate, a core layer that extends along alongitudinal direction and through which light can propagate, and asupport formed from a material with a smaller refractive index than thecore layer and configured to connect at least a portion of the substrateand at least a portion of the core layer to support the core layer withrespect to the substrate. At least a portion of the core layer isprovided in a manner allowing contact with a gas or a liquid. Aconnecting portion of the support connected to the core layer is shiftedfrom a position having a shortest distance from a center to an outersurface in a cross-section perpendicular to the longitudinal directionof the core layer. The longitudinal direction is the direction ofgreatest extension in a three-dimensional structure that is elongated inat least one direction. The longitudinal direction may refer not only toa linear direction but also to a curved direction. The cross-sectionperpendicular to the longitudinal direction of the core layer may be,but is not limited to, a rectangle. The cross-section may have any shapethat is not circular and for which the distance from the center of thecross-section to the outer surface varies with rotation about an axis atthe center of the cross-section. The refractive index refers to therefractive index relative to light of any wavelength or light of aparticular wavelength. Particularly in an optical concentrationmeasuring apparatus, the light of a particular wavelength is the lightpropagating through the core layer. The width direction is a directionperpendicular to the longitudinal direction of the core layer andparallel to the principal surface of the substrate in the presentembodiment. The principal surface of the substrate is a surfaceperpendicular to the thickness direction of the substrate and refers, inthe present embodiment, to the surface with the greatest area among thesix surfaces forming the substrate. At least a portion of the core layermay be provided in a manner allowing contact, via a film that is thinnerthan the wavelength of light propagating through the core layer, withthe gas to be measured or the liquid to be measured.

The connecting portion of the support connected to the core layer in theoptical waveguide according to the second embodiment is shifted from theposition having the shortest distance from the center to the outersurface in a cross-section perpendicular to the longitudinal directionof the core layer. In other words, in a plane orthogonal to thelongitudinal direction that becomes the direction along which lightpropagates, the connecting portion is provided at a position fartheraway than the position closest to the center through which light mainlypropagates. Consequently, the optical waveguide according to the secondembodiment can expand the region of interaction between an evanescentwave and the gas or liquid to be measured while decreasing absorption ofthe evanescent wave by the support. Therefore, the optical waveguideaccording to the second embodiment can increase the measurementsensitivity with respect to a gas or liquid around the core layer.

In the first embodiment and the second embodiment, the support mayinclude a first support and a second support. Connecting portions of thefirst support and the second support connected to the core layer areshifted from the position having the shortest distance from the centerto the outer surface in a cross-section perpendicular to thelongitudinal direction of the core layer, and in a width direction ofthe core layer, a connecting portion of the first support is positionedbetween the center and one end, and a connecting portion of the secondsupport is positioned between the center and the other end. Theconnecting portion of the first support and the connecting portion ofthe second support may be intermittently present along the longitudinaldirection, or the connecting portion of the first support and theconnecting portion of the second support may be alternately presentalong the longitudinal direction. At least a portion of the core layermay be exposed or covered by a thin film. In an optical concentrationmeasuring apparatus, the thin film is thinner than the wavelength oflight propagating through the core layer.

In the optical waveguide with this structure, the connecting portions ofthe first support and the second support connected to the core layer areshifted from the position having the shortest distance from the centerto the outer surface in a cross-section perpendicular to thelongitudinal direction of the core layer. In other words, in a planethat includes the support and is orthogonal to the longitudinaldirection that becomes the direction along which light propagates, theconnecting portions are provided at positions farther away than theposition closest to the center through which light mainly propagates.Absorption of the evanescent wave by the first support and the secondsupport can therefore be suppressed. At the same time, since the corelayer includes a region connected to neither the first support nor thesecond support, the region of interaction between an evanescent wave andthe gas or liquid to be measured can be expanded while the propagationloss of light for measurement is reduced. Furthermore, the core layerhas good symmetry on a macro level as a result of the connecting portionof the first support being positioned between the center and one end,and the connecting portion of the second support being positionedbetween the center and the other end, in the width direction of the corelayer. This can improve the mechanical strength of the core layersupported on the substrate. Therefore, the measurement sensitivity canbe improved while maintaining high mechanical strength in an opticalconcentration measuring apparatus including the present opticalwaveguide. The connecting portion of the first support and theconnecting portion of the second support are intermittently presentalong the longitudinal direction. In an optical waveguide configured sothat the support is intermittently present, no predetermined layer isprovided between the substrate and the core layer, except in a regionnecessary for supporting the core layer. In other words, a space isformed between the substrate and the core layer except in a region wherethe support is provided in an optical waveguide configured in this way.The region of interaction between the substance to be measured and theevanescent wave can thereby be expanded, and absorption of light L bythe material provided on the substrate side of the core layer (forexample, material configuring the support) can be reduced. In otherwords, more of the outer surface is not in contact with the firstsupport or the second support in a portion of the core layer in thelongitudinal direction, which further expands the region of interactionbetween the evanescent wave and the substance to be measured and furtherimproves the sensitivity of a sensor using the optical waveguide. Theconnecting portion of the first support and the connecting portion ofthe second support are alternately present along the longitudinaldirection. The optical waveguide therefore has even better symmetry withrespect to the longitudinal direction, and the mechanical strength isfurther improved.

An optical waveguide according to a third embodiment of the presentdisclosure is an optical waveguide for use in an optical concentrationmeasuring apparatus for measuring concentration of a gas to be measuredor a liquid to be measured, the optical waveguide including a corelayer, which extends along a longitudinal direction and through whichlight can propagate, and a protective film that is formed on at least aportion of a surface of the core layer, has a thickness of 1 nm or moreand less than 20 nm, and has a smaller refractive index than the corelayer. In a cross-section of at least a portion perpendicular to thelongitudinal direction of the core layer, the entire surface of the corelayer is not exposed. The longitudinal direction is the direction ofgreatest extension in a three-dimensional structure that is elongated inat least one direction. The longitudinal direction may refer not only toa linear direction but also to a curved direction. The width directionis a direction perpendicular to the longitudinal direction of the corelayer and parallel to the principal surface of the substrate in thepresent embodiment. The principal surface of the substrate is a surfaceperpendicular to the thickness direction of the substrate and is thesurface with the greatest area.

In a variety of modes of use of a sensor using the ATR method, asubstance to be detected is detected sporadically or continually atcertain intervals. Optical waveguides and optical concentrationmeasuring apparatuses are therefore required to be capable of detectinga substance to be detected with high sensitivity while also preventingage-related degradation in sensitivity.

In the optical waveguide according to the third embodiment, a protectivefilm with a smaller refractive index than the core layer is formed on atleast a portion of the surface of the core layer, the protective filmhas a thickness of 1 nm or more and less than 20 nm, and in across-section perpendicular to the longitudinal direction of the corelayer, the entire surface of the core layer is not exposed. This canprevent a change in the surface condition of the core layer withoutgreatly decreasing the amount of interaction between the evanescent waveextending from the core layer and the gas or liquid to be measured.Consequently, a reduction in the detection sensitivity of the opticalconcentration measuring apparatus that includes the optical waveguideaccording to the third embodiment can be suppressed.

An optical waveguide according to a fourth embodiment of the presentdisclosure includes a core layer, which extends along a longitudinaldirection and through which light can propagate, and a protective filmthat is formed on at least a portion of a surface of the core layer, hasa thickness of 1 nm or more and less than 20 nm, and has a smallerrefractive index than the core layer. At least a portion of theprotective layer is provided in a manner allowing contact with a gas ora liquid. In a cross-section of at least a portion perpendicular to thelongitudinal direction of the core layer, the entire surface of the corelayer is not exposed.

In the optical waveguide according to the fourth embodiment, aprotective film with a smaller refractive index than the core layer isformed on at least a portion of the surface of the core layer, theprotective film has a thickness of 1 nm or more and less than 20 nm, andin a cross-section perpendicular to the longitudinal direction of thecore layer, the entire surface of the core layer is not exposed. Thiscan prevent a change in the surface condition of the core layer withoutgreatly decreasing the amount of interaction between the evanescent waveextending from the core layer and the gas or liquid.

An optical waveguide according to a fifth embodiment of the presentdisclosure is an optical waveguide for use in an optical concentrationmeasuring apparatus for measuring concentration of a gas to be measuredor a liquid to be measured, the optical waveguide including a corelayer, which extends along a longitudinal direction and through whichlight can propagate, and a protective film that is formed on at least aportion of a surface of the core layer, has a thickness less than thewavelength of light propagating through the core layer, includesnitrogen, and has a smaller refractive index than the core layer. In across-section of at least a portion perpendicular to the longitudinaldirection of the core layer, the entire surface of the core layer is notexposed.

In the optical waveguide according to the fifth embodiment, a protectivefilm including nitrogen and having a smaller refractive index than thecore layer is formed on at least a portion of the surface of the corelayer, and in a cross-section perpendicular to the longitudinaldirection of the core layer, the entire surface of the core layer is notexposed. This can suppress oxidation of the core layer and prevent achange in the surface condition of the core layer. Consequently, areduction in the detection sensitivity of the optical concentrationmeasuring apparatus that includes the optical waveguide according to thefifth embodiment can be suppressed.

An optical waveguide according to a sixth embodiment of the presentdisclosure includes a core layer, which extends along a longitudinaldirection and through which light can propagate, and a protective filmthat is formed on at least a portion of a surface of the core layer, hasa thickness less than the wavelength of light propagating through thecore layer, includes nitrogen, and has a smaller refractive index thanthe core layer. At least a portion of the protective layer is providedin a manner allowing contact with a gas or a liquid. In a cross-sectionof at least a portion perpendicular to the longitudinal direction of thecore layer, the entire surface of the core layer is not exposed.

In the optical waveguide according to the sixth embodiment, a protectivefilm including nitrogen and having a smaller refractive index than thecore layer is formed on at least a portion of the surface of the corelayer, and in a cross-section perpendicular to the longitudinaldirection of the core layer, the entire surface of the core layer is notexposed. This can suppress oxidation of the core layer and prevent achange in the surface condition of the core layer.

The constituent elements of the optical waveguide are described belowwith examples.

<Core Layer>

The core layer may be any layer that extends in the longitudinaldirection and through which light can propagate in the longitudinaldirection. Specific examples include core layers made of silicon (Si) orgallium arsenide (GaAs). Furthermore, the effects of the thirdembodiment through the sixth embodiment are easier to achieve when thecore layer is made of a material that does not include nitrogen. Thelongitudinal direction is the direction of greatest extension in athree-dimensional structure that is elongated in at least one direction.The longitudinal direction may refer not only to a linear direction butalso to a curved direction. A vertical cross-section at any positionalong the longitudinal direction of the core layer is not circular butrather has any shape, such as a rectangle, for which the distance fromthe center of the cross-section to the outer surface varies withrotation about an axis at the center of the cross-section. Accordingly,the core layer has an elongated plate shape in the present embodiment.

At least a portion of the core layer may, for example, be exposed toallow direct contact with a gas to be measured or a liquid to bemeasured. At least a portion of the core layer may, for example, becoated with a thin film that is thinner than the wavelength of lightpropagating through the core layer to allow contact, via the thin film,with a gas to be measured or a liquid to be measured. This allows theevanescent wave to interact with the gas to be measured or the liquid tobe measured to allow measurement of the concentration of the gas to bemeasured or the liquid to be measured.

The surface of the core layer need not include an exposed region in avertical cross-section in the longitudinal direction of the core layer,which is the propagation direction of light. Due to natural oxidation,for example, the surface state of an exposed region worsens over time.Accordingly, in a cross-section perpendicular to the longitudinaldirection of the core layer, a support and a protective film, describedbelow, are preferably present without the surface of the core layerbeing exposed, or the protective film is preferably formed around theentire surface of the core layer.

At least a portion of the core layer may be floating, without beingjoined to the below-described support. This allows an increase in theamount of interaction between the evanescent wave extending from thecore layer and the surrounding gas or liquid.

The below-described support need not be present in the entire regionbetween the core layer and the substrate in a cross-sectionperpendicular to the longitudinal direction in at least a portion in thelongitudinal direction of the core layer. This allows an increase in theamount of interaction between the evanescent wave extending out from thecore layer and the surrounding gas or liquid. Stating that the supportis not present refers to the core layer forming a bridge between twosupports that are adjacent in the longitudinal direction. Furthermore,stating that the support is not present refers to how the entire regionbetween the core layer and the substrate includes a space, or a mediumthat has a lower absorption of light propagating through the core layerthan the support does, between two supports that are adjacent in thelongitudinal direction.

The light propagating through the core layer may be infrared lightserving as an analog signal. Infrared light serving as an analog signaldoes not refer to determining the change in the energy of light to beone of two values, i.e. 0 (low level) or 1 (high level), but rather to asignal that carries the amount of change in the energy of light. Theoptical waveguide according to each embodiment can therefore be appliedto sensors or to analysis apparatuses. In this case, the wavelength ofthe infrared light may be from 2 μm or more to 10 μm or less. This is awavelength at which representative gasses that float in the environment(CO₂, CO, NO, N₂O, SO₂, CH₄, H₂O, and the like) are absorbed.Consequently, the optical waveguide according to each embodiment can beused as a gas sensor.

<Substrate>

The substrate of the optical waveguide according to the first embodimentand the second embodiment may be any substrate on which the support andthe core layer can be formed. The optical waveguide according to thethird embodiment through the sixth embodiment may further include asubstrate. The substrate of the optical waveguide according to the thirdembodiment through the sixth embodiment may be any substrate on whichthe core layer can be formed. Specific examples include a siliconsubstrate and a GaAs substrate. The principal surface of the substrateindicates the surface in a horizontal direction (a directionperpendicular to the film thickness direction) of the substrate.

<Support>

The support of the optical waveguide according to the first embodimentand the second embodiment connects at least a portion of the substratewith at least a portion of the core layer. The optical waveguideaccording to the third embodiment through the sixth embodiment mayfurther include a support. The support of the optical waveguideaccording to the third embodiment through the sixth embodiment may beany support capable of joining the substrate and the core layer. Thesupport supports the core layer with respect to the substrate. Thesupport may be any material that has a smaller refractive index than thecore layer, with respect to light of any wavelength or light propagatingthrough the core layer, and that is capable of joining the substrate andthe core layer. Examples of the material forming the support includeSiO₂. The connecting portion of the support connected to the core layeris shifted from the position having the shortest distance from thecenter to the outer surface in a cross-section perpendicular to thelongitudinal direction of the core layer (in the present embodiment, thecentral position in the width direction of the core layer, which has arectangular cross-section). The support may be structured to bediscontinuous. In other words, the optical waveguide may include afloating core layer having a region in which the support is not present.

The support may include a first support and a second support. Theconnecting portion of the first support to the core layer is positionedbetween the center and one end in the width direction of the core layer,and the connecting portion of the second support to the core layer ispositioned between the center and the other end in the width directionof the core layer. The connecting portions of the first support and thesecond support may be intermittently present along the longitudinaldirection of the core layer. The connecting portions of the firstsupport and the second support may be alternately present along thelongitudinal direction of the core layer. The connecting portions of thefirst support and the second support may be shaped to expand in thelongitudinal direction of the core layer with proximity to the center ofthe core layer from an edge of the core layer in the width direction ofthe core layer. With this shape, the conditions around the core layerchange gradually along the longitudinal direction of the core layer froma region without the first support or the second support to an area withthe first support or the second support (or vice versa). A sudden changein the surrounding conditions for the light propagating through the corelayer can therefore be suppressed, thus allowing a reduction in thescattering loss of light propagating through the core layer.

An example of a method of forming the support is to etch a buried oxide(BOX) layer (SiO2 layer) of a silicon on insulator (SOI) substrate,thereby forming a structure in which the BOX layer supports the corelayer (Si layer) with respect to the substrate (Si layer).

The core layer may be divided into a plurality of portions. In thiscase, the connecting portion of the support may include a plurality ofspatially separated connecting portions. One connecting portion amongthe plurality of connecting portions may be connected to one portionamong the plurality of portions of the divided core layer, and anotherconnecting portion among the plurality of connecting portions may beconnected to another portion among the plurality of portions of thedivided core layer. A plurality of core layers may be provided. In thiscase, one connecting portion among the plurality of connecting portionsmay be connected to one core layer among the plurality of core layers,and another connecting portion among the plurality of connectingportions may be connected to another core layer among the plurality ofcore layers.

A plurality of supports may be provided. At least one support among theplurality of supports may include a plurality of spatially separatedconnecting portions, as described above. This configuration, with onesupport connected to a plurality of portions of a core layer or aplurality of core layers, allows the support to be formed efficientlyover a small area.

<Protective Film>

The core layer in an optical waveguide according to the first embodimentand the second embodiment may further include a protective film that isformed on at least a portion of the surface of the core layer and eitherhas a film thickness of 1 nm or more and less than 20 nm or has asmaller refractive index than the core layer. The protective layer ofthe optical waveguide according to the third embodiment through thesixth embodiment may be any protective layer that can be formed on thecore layer and that has a smaller refractive index than the core layer.The protective layer according to the third embodiment through the sixthembodiment may have a film thickness of 1 nm or more and less than 20nm. Specific examples include a protective film formed by a siliconnitride film or a silicon oxynitride film. The protective film may be asingle-layer film or a multi-layer film. This can prevent a change inthe surface condition of the core layer without greatly decreasing theamount of interaction between the evanescent wave extending from thecore layer and the gas or liquid surrounding the core layer.

A thickness of 1 nm or more for the protective film can suppress theformation of a natural oxide film on the surface of the core layer. Athickness of less than 20 nm for the protective film does not greatlydecrease the amount of interaction between the evanescent wave extendingfrom the core layer and the gas or liquid surrounding the core layer.The lower limit on the thickness of the protective film may be 2 nm, andthe upper limit on the thickness of the protective film may be 5 nm.

The protective film may include nitrogen. This allows furthersuppression of oxidation of the core layer. The film including nitrogenmay be a single-layer film or may be a laminate of a film includingnitrogen and a film not including nitrogen. The effect of deterringoxidation increases as the nitrogen content of the protective film ishigher. The protective film may be a film that includes nitrogen and hasa nitrogen content of 1% or more in at least a partial region of thefilm.

For example, when the core layer is formed from silicon, the material ofthe protective film may be a silicon nitride film, a silicon oxide film,or a silicon oxynitride film. A film including nitrogen has the effectof suppressing oxidation. A silicon nitride film, a silicon oxide film,and a silicon oxynitride film have a sufficiently smaller refractiveindex than silicon and are therefore excellent materials for forming acladding layer. In particular, a silicon nitride film and a siliconoxynitride film exhibit little absorption of infrared light. Therefore,formation of a protective film on the surface of the core layer cansuppress a reduction in the detection sensitivity of the gas to bemeasured or the liquid to be measured.

Here, when a substance such as silicon is released in the air, a siliconoxide film may form naturally on the surface. This natural oxide film isless than 1 nm thick and does not include nitrogen. Hence, the naturaloxide film is distinguished from the protective film in the presentdisclosure with regard to these points.

At least a portion of the protective film may be provided in a mannerallowing contact with the gas or liquid of which the concentration is tobe measured by the optical concentration measuring apparatus. This canincrease the amount of interaction between the evanescent wave extendingfrom the core layer and the gas to be measured or liquid to be measuredas compared to when at least a portion of the protective film is not incontact with the gas to be measured or the liquid to be measured. Inother words, when at least a portion of the protective film is providedin a manner allowing contact with the gas to be measured or the liquidto be measured, a decrease in the amount of interaction between theevanescent wave extending from the core layer and the gas to be measuredor liquid to be measured can be prevented.

The protective film may be formed around the entire surface of at leasta portion of the core layer in a cross-section perpendicular to thelongitudinal direction of the core layer, i.e. perpendicular to thepropagation direction of light. In an optical waveguide having afloating core layer including the region with no support present,deterioration of the core layer can effectively be suppressed by formingthe protective film around the entire surface of the floating corelayer.

Methods such as deposition by thermal chemical vapor deposition (CVD) oroxidation can be used as the method of forming the protective film. Theprotective film can be formed by deposition using thermal CVD in thecase of a silicon nitride film and can be formed by oxidation under anatmosphere including NO or N₂O in the case of a silicon oxynitride film.The protective film can be formed around the entire surface of the corelayer by thermal CVD or oxidation.

<Optical Concentration Measuring Apparatus>

An optical concentration measuring apparatus according to embodiments ofthe present disclosure includes the optical waveguide according toembodiments of the present disclosure, a light source capable of causinglight to enter the core layer, and a detector capable of detecting lightthat has propagated through the core layer.

The constituent elements of the optical concentration measuringapparatus are described below with examples.

<Light Source>

The light source may be any light source capable of causing light toenter the core layer. An incandescent bulb, a ceramic heater, a microelectro mechanical systems (MEMS) heater, an infrared light emittingdiode (LED), or the like can be used as the light source in the case ofusing infrared light to measure a gas. A mercury lamp, an ultravioletLED, or the like can be used as the light source in the case of usingultraviolet rays to measure a gas. An electron beam, an electron laser,or the like can be used as the light source in the case of using x-raysto measure a gas.

The light propagating through the core layer of the optical waveguideprovided in the optical concentration measuring apparatus may beinfrared light serving as an analog signal. Infrared light serving as ananalog signal does not refer to determining the change in the energy oflight to be one of two values, i.e. 0 (low level) or 1 (high level), butrather to a signal that carries the amount of change in the energy oflight. The optical concentration measuring apparatus can therefore beapplied to sensors or to analysis apparatuses. In this case, thewavelength of the infrared light may be from 2 μm or more to 10 μm orless. This is a wavelength at which representative gasses that float inthe environment (CO₂, CO, NO, N₂O, SO₂, CH₄, H₂O, and the like) areabsorbed. Consequently, the optical concentration measuring apparatus ofthe present embodiment can be used as a gas sensor.

<Detector>

The detector may be any detector capable of detecting light that haspropagated through the core layer of the optical waveguide. A thermalinfrared sensor such as a pyroelectric sensor, a thermopile, or abolometer; a quantum infrared sensor such as a diode or aphototransistor; or the like can be used as the detector in the case ofusing infrared light to measure a gas. A quantum ultraviolet sensor,such as a diode or a phototransistor, or the like can be used in thecase of using ultraviolet rays to measure a gas. Various semiconductorsensors can be used as the detector in the case of using x-rays tomeasure a gas.

FIRST EMBODIMENT

An optical waveguide and an optical concentration measuring apparatusaccording to the first embodiment of the present disclosure aredescribed with reference to FIG. 1 through FIG. 6.

FIG. 1 and FIG. 2 illustrate the schematic configuration of an opticalconcentration measuring apparatus 1 according to the first embodimentand are also conceptual drawings of the ATR method using the opticalwaveguide 10 according to the first embodiment.

As illustrated in FIG. 1, the optical concentration measuring apparatus1 is installed and used in an exterior space 2 containing a gas whoseconcentration or the like is to be detected. The optical concentrationmeasuring apparatus 1 includes the optical waveguide 10 according to thefirst embodiment, a light source 20 capable of causing light (infraredlight IR in the first embodiment) to enter a core layer 11 provided inthe optical waveguide 10, and a light detector (an example of adetector) 40 capable of detecting the infrared light IR that haspropagated through the core layer 11.

The optical waveguide 10 includes a substrate 15, the core layer 11through which the infrared light IR (an example of light) can propagate,and a support 17 configured to connect at least a portion of thesubstrate 15 with at least a portion of the core layer 11 and supportthe core layer 11 with respect to the substrate 15. The core layer 11and the substrate 15 are formed from silicon (Si), and the support 17 isformed from silicon dioxide (SiO₂).

The substrate 15 is plate-shaped, for example. The core layer 11 is arectangular parallelepiped, for example. The optical waveguide 10includes a grating coupler 118 formed at one end of the core layer 11 inthe longitudinal direction and a grating coupler 119 formed at the otherend of the core layer 11 in the longitudinal direction (the left-rightdirection in FIG. 1). The grating coupler 118 is disposed in theemission direction of the light source 20 (in the first embodiment,vertically downward in a state such that the stacking direction of theoptical waveguide 10 is parallel to the vertical direction and thesubstrate 15 is disposed to face vertically downward). The gratingcoupler 118 couples the infrared light IR incident from the light source20 with the infrared light IR propagating through the core layer 11. Thegrating coupler 119 is disposed in a direction facing the light detector40 (in the first embodiment, vertically downward in a state such thatthe stacking direction of the optical waveguide 10 is parallel to thevertical direction and the substrate 15 is disposed to face verticallydownward). The grating coupler 119 extracts the infrared light IRpropagating through the core layer 11 and emits the infrared light IRtowards the light detector 40.

As illustrated in FIG. 2, the optical waveguide 10 is structured toinclude a space 13 below the core layer 11, without including apredetermined layer such as a cladding layer, except in a region wherethe support 17 is provided. A connecting portion 171 of the support 17connected to the core layer 11 is shifted from the position NP havingthe shortest distance from the center to the outer surface in across-section perpendicular to the longitudinal direction of the corelayer 11 (in the first embodiment, the central position in the widthdirection of the cross-section). In other words, the connecting portion171 of the support 17 is positioned towards one end (the right end inFIG. 2) from the center of the core layer 11 in the width direction (theleft-right direction in FIG. 2).

Here, the effects of the optical waveguide 10 according to the firstembodiment are described by comparison with an optical waveguide 10′that has the conventional pedestal structure illustrated in FIG. 36.

A sensor using the ATR method often uses single-mode propagation oflight inside the core layer. In the optical concentration measuringapparatus 1 according to the first embodiment as well, light (infraredlight) propagates in single-mode inside the core layer 11 provided inthe optical waveguide 10. In the case of multi-mode propagation as well,however, the effects of the present disclosure are obtained, since alight component propagating through the center of the core layer 11 ispresent. As illustrated in FIG. 2, when infrared light IR propagatesthrough the core layer 11 in single-mode, the optical axis OA of theinfrared light IR is positioned nearly at the center of the core layer11 in a cross-section along a plane orthogonal to the longitudinaldirection, which is the propagation direction of the infrared light IR.At this time, an evanescent wave EW extending around the core layer 11increases near the surface of the core layer 11 close to the opticalaxis OA, reaching a maximum near the outer surface where the distancefrom the center of the core layer 11, which overlaps with the opticalaxis OA, is shortest. In FIG. 36, the distribution of the evanescentwave EW of the infrared light IR propagating through the core layer 11′of the optical waveguide 10′ with a conventional structure is similar tothat of the optical waveguide 10 of the first embodiment.

In a sensor using the ATR method, the sensitivity of the sensor israised by expanding the region of interaction between the evanescentwave extending from the core layer and the substance to be measured anddecreasing the absorption of light by material other than the substanceto be measured (i.e., absorption of light by the support or the like).In the structure illustrated in FIG. 36, however, the connecting portionconnecting the core layer and the support for supporting the core layeris positioned near the outer surface where the distance from the centerof the core layer is shortest in a cross-section along a planeorthogonal to the propagation direction of light through the core layer.The support therefore ends up overlapping with the area of the outersurface where the distance from the optical axis of light undergoingsingle-mode propagation is shortest. The evanescent wave extendingaround the core layer is greatest by the surface near the optical axis.Hence, if the support is near this outer surface, much of the evanescentwave is absorbed by the material forming the support. A sensor using anoptical waveguide with such a structure thus has the problem ofdecreased detection sensitivity of the substance to be measured.

This problem with a conventional optical waveguide is now described withreference to FIG. 36. As illustrated in FIG. 36, the support 17′ in theoptical waveguide 10′ with a conventional pedestal structure is providedbetween the center of the core layer 11′ and the substrate 15′ within aplane (i.e. the cross-section illustrated in FIG. 36) orthogonal to thelongitudinal direction, which is the propagation direction of light L.When the connecting portion connecting the core layer 11′ and thesupport 17′ for supporting the core layer 11′ is thus positioned at thewidthwise center of the core layer 11′ in a cross-section along a planeorthogonal to the propagation direction of light L, the evanescent waveEW may be absorbed by the material forming the support 17′, and theregion of the support 17′ may become an obstacle that reduces the regionof interaction between the evanescent wave EW and the substance to bemeasured. Consequently, the sensitivity of a sensor using the opticalwaveguide 10′ decreases.

As illustrated in FIG. 2, the optical waveguide 10 according to thefirst embodiment is structured so that the core layer 11 is supported bythe support 17 with respect to the substrate 15 while the space 13 isformed between the core layer 11 and the substrate 15, like theconventional optical waveguide 10′. The core layer 11 has a symmetricalstructure about the center in a cross-section perpendicular to thelongitudinal direction. When the propagation of the infrared light IRthrough the core layer 11 is single-mode, the optical axis OA of theinfrared light IR propagating through the core layer 11 is in the centerof the core layer 11. As illustrated in FIG. 2, the support 17 istherefore shifted to one side from the widthwise center of the corelayer 11 (to the right in FIG. 2). The support 17 can thereby be removedfrom the region where the evanescent wave EW is most concentrated. Inother words, the connecting portion 171 of the support 17 connecting tothe core layer 11 is not at the position, near the outer surface, withthe shortest distance from the center of the core layer 11 in across-section perpendicular to the longitudinal direction. The opticalwaveguide 10 thus becomes an optical waveguide that passes an analogsignal through the core layer 11 that is partially floating.Accordingly, the optical concentration measuring apparatus 1 thatincludes the optical waveguide 10 can prevent a decrease, due to thepresence of the support 17, in the detection characteristics of thesubstance to be measured insofar as possible.

Next, a method of manufacturing the optical waveguide 10 according tothe first embodiment is described using FIGS. 3A, 3B, 4A, 4B, 5A, 5B,6A, and 6B, with reference to FIG. 1 and FIG. 2. FIG. 3A is amanufacturing process plan view of the optical waveguide 10, and FIG. 3Bis a cross-sectional manufacturing process end view of the opticalwaveguide 10 along the B-B line in FIG. 3A. FIG. 4A is a manufacturingprocess plan view of the optical waveguide 10, and FIG. 4B is across-sectional manufacturing process end view of the optical waveguide10 along the B-B line in FIG. 4A. FIG. 5A is a manufacturing processplan view of the optical waveguide 10, and FIG. 5B is a cross-sectionalmanufacturing process end view of the optical waveguide 10 along the B-Bline in FIG. 5A. FIG. 6A is a manufacturing process plan view of theoptical waveguide 10, and FIG. 6B is a cross-sectional manufacturingprocess end view of the optical waveguide 10 along the B-B line in FIG.6A. A plurality of optical waveguide main portions are simultaneouslyformed on one support substrate 15 a and subsequently separated tomanufacture the optical waveguide 10. In FIGS. 3A, 3B, 4A, 4B, 5A, 5B,6A, and 6B, only one optical waveguide main portion among a plurality ofoptical waveguide main portions is illustrated.

First, an SiO₂ film is formed on either or both of the support substrate15 a, which is formed from silicon and ultimately becomes the substrate15, and an active substrate 11 a, which is formed from silicon and fromwhich the core layer 11 is formed. The support substrate 15 a and theactive substrate 11 a are then stuck together, with the SiO₂ filmtherebetween, and thermally treated to be joined. The active substrate11 a is then ground, polished, or the like to a predetermined thicknessto adjust the film thickness of the active substrate 11 a. Consequently,as illustrated in FIGS. 3A, 3B, an SOI substrate 100 is formed to have a“silicon-insulating layer-silicon” structure that includes the supportsubstrate 15 a, a BOX layer 17 a formed on the support substrate 15 a,and the active substrate 11 a formed on the BOX layer 17 a.

Next, lithography and etching are used on the SOI substrate 100 to etchthe active substrate 11 a and form a core layer 11 in the shape of arectangular parallelepiped. Consequently, as illustrated in FIGS. 4A,4B, an optical waveguide main portion 10 a is formed to include theplate-shaped support substrate 15 a, the plate-shaped BOX layer 17 aformed on the support substrate 15 a, and the core layer 11 formed as arectangular prism on a portion of the BOX layer 17 a.

Next, as illustrated in FIGS. 5A, 5B, a mask layer M1 covering a portionof the core layer 11 and the BOX layer 17 a is formed. The mask layer M1is positioned towards one end from the widthwise center of the corelayer 11. The mask layer M1 may be a photoresist or a hard mask such asa silicon nitride film.

Next, a portion of the BOX layer 17 a of the optical waveguide mainportion 10 a is removed by wet etching or the like, with the mask layerM1 as a mask. Consequently, as illustrated in FIGS. 6A, 6B, the support17 is formed at a position towards one side (the right in FIG. 6B) fromthe widthwise center of the core layer 11 (i.e. a position shifted inthe width direction from the optical axis OA of the infrared lightpropagating through the core layer 11). This yields a structure in whicha portion of the core layer 11 is floating. In other words, theconnecting portion 171 of the support 17 connected to the core layer 11is not located at a position, on the outer surface, with the shortestdistance from the center of the core layer 11 (the widthwise center ofthe outer surface when the core layer 11 is elongated widthwise as inFIG. 6B) in a plane orthogonal to the longitudinal direction, which isthe propagation direction of the infrared light. Rather, the connectingportion 171 is formed towards one of the ends from the widthwise centerof the core layer 11. The space 13 is formed between the center of thecore layer 11 and a principal surface 15 s of the substrate 15.

Subsequently, the mask layer M1 is etched. After etching of the masklayer M1, a protective film may be formed on the surface of the corelayer 11. This protective film may be a film including nitrogen, and thefilm thickness may be 1 nm or more and less than 20 nm. Inclusion of theprotective film on the surface of the core layer 11 can preventdeterioration of the surface of the core layer 11, due to naturaloxidation or the like, while maintaining the measurement sensitivity ofthe optical waveguide 10 with respect to the substance to be measured.

Furthermore, the slit-shaped grating coupler 118 is subsequently formedat one end in the longitudinal direction of the core layer 11, and theslit-shaped grating coupler 119 is formed at the other end in thelongitudinal direction of the core layer 11 (see FIG. 1). The gratingcoupler 118 and the grating coupler 119 may be formed simultaneouslywith the core layer 11 or formed earlier than the core layer 11.

Next, the support substrate 15 a is cut in a predetermined region toseparate the optical waveguide main portion 10 a. The optical waveguide10 with the support 17 shifted from the optical axis OA of the infraredlight propagating through the core layer 11 is thus completed (see FIG.2).

Furthermore, as illustrated in FIG. 1, the light source 20 is installedto be capable of emitting the infrared light IR onto the grating coupler118 of the optical waveguide 10, and the light detector 40 is disposedto be capable of detecting the infrared light IR emitted from thegrating coupler 119 of the optical waveguide 10, thereby completing theoptical concentration measuring apparatus 1.

SECOND EMBODIMENT

Next, an optical waveguide according to the second embodiment of thepresent disclosure is described with reference to FIG. 7. The opticalwaveguide according to the second embodiment is characterized by use ina region with a densely packed core layer. FIG. 7 is a cross-section ofthe optical waveguide according to the second embodiment in a planeorthogonal to the longitudinal direction, which is the propagationdirection of light.

As illustrated in FIG. 7, an optical waveguide 60 according to thesecond embodiment includes a substrate 15, a core layer 11 formed on thesubstrate 15, and a plurality (two in the second embodiment) of supports17 x, 17 y supporting the core layer 11 with respect to the substrate15. In the second embodiment, the core layer 11 includes a plurality(three in the second embodiment) of separated portions 111, 112, 113.The separated portion 111, separated portion 112, and separated portion113 are arranged in close proximity to each other. The separatedportions 111, 112, 113 are, for example, branches of the core layer 11.

The support 17 x includes a plurality (two in the second embodiment) ofspatially separated connecting portions 171, 172. The connectingportions 171, 172 extend in the longitudinal direction, which is thepropagation direction of light through the core layer 11. The connectingportions 171, 172 are provided at the ends in the width direction on theupper surface of the support 17 x. The connecting portion 171 isprovided at one end, and the connecting portion 172 is provided at theother end. The separated portion 111 is connected to the connectingportion 171, and the separated portion 112 is connected to theconnecting portion 172. The support 17 x connects at least a portion ofthe separated portion 111 and at least a portion of the separatedportion 112 with at least a portion of the substrate 15 to support theseparated portions 111, 112 with respect to the substrate 15.

In a plane that is orthogonal to the longitudinal direction, which isthe propagation direction of light, and that includes the separatedportion 111 of the core layer 11, the connecting portion 171 is notlocated at a position, on the outer surface, with the shortest distancefrom the center of the separated portion 111 of the core layer 11 (thewidthwise center of the outer surface when the separated portion 111 ofthe core layer 11 is elongated widthwise as in FIG. 7). Rather, theconnecting portion 171 is formed towards one of the ends from thewidthwise center of the separated portion 111. Therefore, the opticalwaveguide 60 includes a space 13 a below the separated portion 111 ofthe core layer 11, without including a predetermined layer such as acladding layer.

In a plane that is orthogonal to the longitudinal direction, which isthe propagation direction of light, and that includes the separatedportion 112 of the core layer 11, the connecting portion 172 is notlocated at a position, on the outer surface, with the shortest distancefrom the center of the separated portion 112 of the core layer 11 (thewidthwise center of the outer surface when the separated portion 112 ofthe core layer 11 is elongated widthwise as in FIG. 7). Rather, theconnecting portion 172 is formed towards one of the ends from thewidthwise center of the separated portion 112. Therefore, the opticalwaveguide 60 includes a space 13 b below the separated portion 112 ofthe core layer 11, without including a predetermined layer such as acladding layer.

The support 17 y is provided next to the support 17 x on the connectingportion 172 side of the support 17 x. The separated portion 113 of thecore layer 11 is connected to the connecting portion 173 of the support17 y. The support 17 y connects at least a portion of the separatedportion 113 with at least a portion of the substrate 15 to support theseparated portion 113 with respect to the substrate 15. In a plane thatis orthogonal to the longitudinal direction, which is the propagationdirection of light, and that includes the separated portion 113 of thecore layer 11, the connecting portion 173 is not located at a position,on the outer surface, with the shortest distance from the center of theseparated portion 113 of the core layer 11 (the widthwise center of theouter surface when the separated portion 113 of the core layer 11 iselongated widthwise as in FIG. 7). Rather, the connecting portion 173 isformed towards one of the ends from the widthwise center of theseparated portion 113. Therefore, the optical waveguide 60 includes aspace 13 c below the separated portion 113 of the core layer 11, withoutincluding a predetermined layer such as a cladding layer.

The spaces 13 a, 13 b, 13 c are filled with a substance to be measured,such as a gas or liquid. Consequently, the optical waveguide 60 canincrease the amount of interaction between the substance to be measuredand the evanescent wave at the separated portion 111, the amount ofinteraction between the substance to be measured and the evanescent waveat the separated portion 112, and the amount of interaction between thesubstance to be measured and the evanescent wave at the separatedportion 113. Furthermore, the optical waveguide 60 can reduce theabsorption of infrared light by the supports 17 x, 17 y. The sensitivityof the optical concentration measuring apparatus using the opticalwaveguide 60 therefore improves. This configuration, with one support 17x connected to two separated portions 111, 112 among the plurality ofseparated portions 111, 112, 113 of the core layer 11, also allows thesupports 17 x, 17 y to be formed efficiently over a small area.

The optical waveguide 60 according to the second embodiment includes thecore layer 11 that has three separated portions 111, 112, 113, but thisconfiguration is not limiting. The optical waveguide 60 may include aplurality (three in the second embodiment) of core layers. For example,in this case, two adjacent core layers among the three core layers areconnected to the connecting portions 171, 172 of the support 17 x, andthe remaining core layer is connected to the connecting portion 173 ofthe support 17 y. Similar effects as when the optical waveguide 60includes the core layer 11 having three separated portions 111, 112, 113are thus obtained. In another example, the optical waveguide 60 may havea layout in which one long optical waveguide is folded back. Twoadjacent core layer sections, in a region where three sections of thefolded back core layer are in a row, are connected to the connectingportions 171, 172 of the support 17 x, and the remaining core layersection is connected to the connecting portion 173 of the support 17 y.Similar effects as when the optical waveguide 60 includes the core layer11 having three separated portions 111, 112, 113 are thus obtained.

A method of manufacturing the optical waveguide 60 according to thesecond embodiment is similar to that of the optical waveguide 10according to the first embodiment, except for a difference in the shapeof the core layer 11 and the mask layer for forming the supports 17 x,17 y. Hence, a description thereof is omitted.

THIRD EMBODIMENT

Next, an optical waveguide according to the third embodiment of thepresent disclosure is described with reference to FIGS. 8, 9A, 9B, 10A,and 10B. The optical waveguide according to the third embodiment ischaracterized by multi-mode propagation of light through the core layer.First, the schematic configuration of an optical waveguide 70 accordingto the third embodiment is described with reference to FIG. 8.

As illustrated in FIG. 8, the optical waveguide 70 includes a substrate15, a core layer 11 provided on the substrate 15, and a support 17configured to connect at least a portion of the substrate 15 with atleast a portion of the core layer 11 and support the core layer 11 withrespect to the substrate 15. The optical waveguide 70 is configured forlight (infrared light in the third embodiment) to propagate through thecore layer 11 along a plurality (three in the third embodiment) ofoptical axes OA1, OA2, OA3.

The connecting portion 171 of the support 17 connected to the core layer11 is provided between the three optical axes OA1, OA2, OA3 of light(infrared light in the third embodiment) undergoing multi-modepropagation. In the third embodiment, the connecting portion 171 of thesupport 17 is provided between the optical axis OA2 and the optical axisOA3. In this way, the connecting portion 171 of the support 17 connectedto the core layer 11 is not provided between the optical axes OA1, OA2,OA3 of light and a principal surface 15 s of the substrate 15. Theoptical axis OA2 nearly matches the center of the core layer 11.Therefore, the connecting portion 171 of the support 17 connected to thecore layer 11 is not located at a position, on the outer surface, withthe shortest distance from the center of the core layer 11 (thewidthwise center of the outer surface when the core layer 11 iselongated widthwise as in FIG. 8) in a plane orthogonal to thelongitudinal direction, which is the propagation direction of light.Rather, the connecting portion 171 is formed towards one of the endsfrom the widthwise center of the core layer 11.

The optical waveguide 70 thus includes spaces 13 a, 13 b filled with asubstance to be measured MO. The space 13 a is provided between theprincipal surface 15 s of the substrate 15 and the optical axes OA1,OA2, along which infrared light propagates through the core layer 11.The space 13 b is provided between the principal surface 15 s of thesubstrate 15 and the optical axis OA3 of the light. The support 17 inthe optical waveguide 70 can thereby be removed from the regions wherethe evanescent wave EW is most concentrated. Accordingly, the opticalwaveguide 70 can prevent a decrease, due to the support 17, in thedetection characteristics of the substance to be measured MO.

Next, a method of manufacturing the optical waveguide 70 according tothe third embodiment is described using FIGS. 9A, 9B, 10A, and 10B, withreference to FIGS. 3A, 3B, 4A, 4B, and 8. FIG. 9A is a manufacturingprocess plan view of the optical waveguide 70, and FIG. 9B is across-sectional manufacturing process end view of the optical waveguide70 along the C-C line in FIG. 9A. FIG. 10A is a manufacturing processplan view of the optical waveguide 70, and FIG. 10B is a cross-sectionalmanufacturing process end view of the optical waveguide 70 along the C-Cline in FIG. 10A. A plurality of optical waveguide main portions aresimultaneously formed on one support substrate 15 a and subsequentlyseparated to manufacture the optical waveguide 70. In FIGS. 9A, 9B, 10A,and 10B, only one optical waveguide main portion among a plurality ofoptical waveguide main portions is illustrated.

First, as in the first embodiment, an SiO₂ film is formed on either orboth of the support substrate 15 a, which is formed from silicon andultimately becomes the substrate 15, and an active substrate 11 a, whichis formed from silicon and from which the core layer 11 is formed. Thesupport substrate 15 a and the active substrate 11 a are then stucktogether, with the SiO₂ film therebetween, and thermally treated to bejoined. The active substrate 11 a is then ground, polished, or the liketo a predetermined thickness to adjust the film thickness of the activesubstrate 11 a. Consequently, as illustrated in FIGS. 3A, 3B, an SOIsubstrate 100 is formed to have a “silicon-insulating layer-silicon”structure that includes the support substrate 15 a, a BOX layer 17 aformed on the support substrate 15 a, and the active substrate 11 aformed on the BOX layer 17 a.

Next, as in the first embodiment, lithography and etching are used onthe SOI substrate 100 to etch the active substrate 11 a and form a corelayer 11 in the shape of a rectangular parallelepiped. Consequently, asillustrated in FIGS. 4A, 4B, an optical waveguide main portion 70 a(corresponding to the optical waveguide main portion 10 a of FIG. 4) isformed to include the plate-shaped support substrate 15 a, theplate-shaped BOX layer 17 a formed on the support substrate 15 a, andthe core layer 11 formed as a rectangular prism on a portion of the BOXlayer 17 a.

Next, as illustrated in FIGS. 9A, 9B, a mask layer M2 is formed to havea center line Cm arranged among axes a1, a2, a3 from which the opticalaxes OA1, OA2, OA3 will be formed. The mask layer M2 may be aphotoresist or a hard mask such as a silicon nitride film.

Next, a portion of the BOX layer 17 a of the optical waveguide mainportion 70 a is removed by wet etching or the like, with the mask layerM2 as a mask. Consequently, as illustrated in FIGS. 10A, 10B, thesupport 17 is formed between the axes a2, a3, from which the opticalaxes OA2, OA3 will be formed, at a position in the width directioncorresponding to the center line Cm of the mask layer M2. This yields astructure in which a portion of the core layer 11 is floating. In otherwords, the connecting portion 171 of the support 17 connected to thecore layer 11 is not located at a position, on the outer surface, withthe shortest distance from the center of the core layer 11 (thewidthwise center of the outer surface when the core layer 11 iselongated widthwise as in FIG. 10B) in a plane orthogonal to thelongitudinal direction, which is the propagation direction of light(infrared light in the third embodiment). Rather, the connecting portion171 is formed towards one of the ends from the widthwise center of thecore layer 11. The connecting portion 171 of the support 17 connected tothe core layer 11 is therefore not provided between the principalsurface 15 s of the substrate 15 connected to the support 17 and theaxes a2, a3 corresponding to the optical axes OA2, OA3 of light. A space13 a forms between the axes a1, a2, which will become the optical axesOA1, OA2 of multi-mode light propagating through the core layer 11, andthe principal surface 15 s of the substrate 15. A space 13 b formsbetween the axis a3, which becomes the optical axis OA3 of the light,and the principal surface 15 s of the substrate 15.

Subsequently, the mask layer M2 is etched. After etching of the masklayer M2, a protective film may be formed on the surface of the corelayer 11, as in the first embodiment. This protective film may be a filmincluding nitrogen, and the film thickness may be 1 nm or more and lessthan 20 nm. Inclusion of the protective film on the surface of the corelayer 11 can prevent deterioration of the surface of the core layer 11,due to natural oxidation or the like, while maintaining the measurementsensitivity of the optical waveguide 70 with respect to the substance tobe measured.

Subsequently, a slit-shaped input-side grating coupler like the gratingcoupler 118 in FIG. 1 is further formed at one end in the longitudinaldirection of the core layer 11, and a slit-shaped output-side gratingcoupler like the grating coupler 119 in FIG. 1 is further formed at theother end in the longitudinal direction of the core layer 11. Thegrating coupler 118 and the grating coupler 119 may be formedsimultaneously with the core layer 11 or formed earlier than the corelayer 11.

Next, the support substrate 15 a is cut in a predetermined region toseparate the optical waveguide main portion 70 a. This completes theoptical waveguide 70 (see FIG. 8), in which the connecting portion 171of the support 17 is positioned between the optical axis OA2 and theoptical axis OA3 of light propagating through the core layer 11.

Furthermore, while omitted from the drawings, a light source 20 isinstalled to be capable of emitting infrared light onto the input-sidegrating coupler of the optical waveguide 70, and a light detector 40 isdisposed to be capable of detecting the infrared light emitted from theoutput-side grating coupler of the optical waveguide 70, therebycompleting an optical concentration measuring apparatus.

In this way, the optical waveguide 70 is structured so that theconnecting portion 171 of the support 17 that supports the core layer 11is shifted in the width direction from the optical axes OA1, OA2, OA3 oflight propagating through the core layer 11. This can prevent adecrease, due to the support 17, in the detection characteristics of thesubstance to be measured MO.

As described above, the first through third embodiments can provide anoptical waveguide and an optical concentration measuring apparatus thathave a support for supporting the core layer without reducing thesensitivity of a sensor.

Furthermore, the optical waveguide according to the first through thirdembodiments can increase the amount of interaction between theevanescent wave of light propagating through the core layer and thesubstance to be measured and can reduce the amount of the evanescentwave that is absorbed by the support. The optical waveguide according tothe first through third embodiments can thereby stably detect thesubstance to be measured, in a variety of modes of use, with highsensitivity.

FOURTH EMBODIMENT

An optical waveguide and an optical concentration measuring apparatusaccording to the fourth embodiment of the present disclosure aredescribed with reference to FIG. 11 through FIG. 26.

FIG. 11 illustrates the schematic configuration of an opticalconcentration measuring apparatus 8 according to the fourth embodimentand is also a conceptual drawing of the ATR method using an opticalwaveguide 80 according to the fourth embodiment.

As illustrated in FIG. 11, the optical concentration measuring apparatus8 is installed and used in an exterior space 2 containing a gas whoseconcentration or the like is to be detected. The optical concentrationmeasuring apparatus 8 includes the optical waveguide 80 according to thefourth embodiment, a light source 20 capable of causing light (infraredlight IR in the fourth embodiment) to enter a core layer 11 provided inthe optical waveguide 80, and a light detector (an example of adetector) 40 capable of detecting the infrared light IR that haspropagated through the core layer 11.

The optical waveguide 80 includes a substrate 15, the core layer 11through which the infrared light IR (an example of light) can propagate,and a first support 87 x and a second support 87 y configured to connectat least a portion of the substrate 15 with at least a portion of thecore layer 11 and discontinuously support the core layer 11 with respectto the substrate 15. The core layer 11 and the substrate 15 are formedfrom silicon (Si), and the first support 87 x and the second support 87y are formed from silicon dioxide (SiO₂).

The substrate 15 is plate-shaped, for example. The core layer 11 is arectangular parallelepiped, for example. The optical waveguide 80includes a grating coupler 118 formed at one end of the core layer 11 inthe longitudinal direction and a grating coupler 119 formed at the otherend of the core layer 11 in the longitudinal direction. The gratingcoupler 118 is disposed in the emission direction of the light source 20(in the fourth embodiment, vertically downward in a state such that thestacking direction of the optical waveguide 10 is parallel to thevertical direction and the substrate 15 is disposed to face verticallydownward). The grating coupler 118 couples the infrared light IRincident from the light source 20 with the infrared light IR propagatingthrough the core layer 11. The grating coupler 119 is disposed in adirection facing the light detector 40 (in the fourth embodiment,vertically downward in a state such that the stacking direction of theoptical waveguide 10 is parallel to the vertical direction and thesubstrate 15 is disposed to face vertically downward). The gratingcoupler 119 extracts the infrared light IR propagating through the corelayer 11 and emits the infrared light IR towards the light detector 40.

FIG. 12 is a cross-sectional end view along the D-D line and F-F line inFIG. 11, and FIG. 13 is a cross-sectional end view along the E-E line inFIG. 11. FIG. 14 is a plan view of the optical waveguide 80, as seenfrom the light source 20 or light detector 40 side, to illustrate thearrangement of the first support 87 x and the second support 87 y.

As illustrated in FIGS. 11, 12, and 13, the optical waveguide 80 isstructured to include a space 13 between the core layer 11 and thesubstrate 15, without including a predetermined layer such as a claddinglayer, except in a region where the first support 87 x or the secondsupport 87 y is provided.

As illustrated in FIG. 12, a connecting portion 871 of the first support87 x connected to the core layer 11 is shifted from the position NPhaving the shortest distance from the center to the outer surface in across-section perpendicular to the longitudinal direction of the corelayer 11 (in the fourth embodiment, the central position in the widthdirection of the cross-section). The connecting portion 871 of the firstsupport 87 x is positioned towards one end (the right end in FIG. 12)from the widthwise center of the core layer 11. As also illustrated inFIG. 11, the connecting portion 871 of the first support 87 x isdiscontinuous in the longitudinal direction.

As illustrated in FIG. 13, a connecting portion 872 of the secondsupport 87 y connected to the core layer 11 is shifted from the positionNP having the shortest distance from the center to the outer surface ina cross-section perpendicular to the longitudinal direction of the corelayer 11 (in the fourth embodiment, the central position in the widthdirection of the cross-section). The connecting portion 872 of thesecond support 87 y is positioned towards the opposite end (the left endin FIG. 13) from the connecting portion 871 of the first support 87 x inthe width direction of the core layer 11. As also illustrated in FIG.11, the connecting portion 872 of the second support 87 y isdiscontinuous in the longitudinal direction.

At least the connecting portions 871, 872 of the first support 87 x andthe second support 87 y are alternately present along the longitudinaldirection of the core layer 11. Furthermore, as illustrated in FIGS. 11and 14, the first support 87 x and the second support 87 y arealternately present along the longitudinal direction of the core layer11.

Here, the effects of the optical waveguide 80 according to the fourthembodiment are described by comparison with the optical waveguide 10′that has the conventional pedestal structure illustrated in FIG. 36.

A sensor using the ATR method often uses single-mode propagation oflight inside the core layer. In the example of the optical concentrationmeasuring apparatus 8 according to the fourth embodiment as well, light(infrared light) propagates in single-mode inside the core layer 11provided in the optical waveguide 80. In the case of multi-modepropagation as well, however, the effects of the present disclosure areobtained, since a light component propagating through the center of thecore layer 11 is present. As illustrated in FIGS. 12 and 13, wheninfrared light IR propagates through the core layer 11 in single-mode,the optical axis OA of the infrared light IR is positioned nearly at thecenter of the core layer 11 in a cross-section along a plane orthogonalto the longitudinal direction, which is the propagation direction of theinfrared light IR. At this time, an evanescent wave EW extending aroundthe core layer 11 increases near the outer surface of the core layer 11close to the optical axis OA, reaching a maximum near the outer surfacewhere the distance from the center of the core layer 11, which overlapswith the optical axis OA, is shortest. In FIG. 36, the distribution ofthe evanescent wave EW of the infrared light IR propagating through thecore layer 11′ of the optical waveguide 10′ with a conventionalstructure is similar to that of the optical waveguide 80 of the fourthembodiment.

In a sensor using the ATR method, the sensitivity of the sensor israised by expanding the region of interaction between the evanescentwave extending from the core layer and the substance to be measured(i.e. expanding the exposed portion of the core layer) and decreasingthe absorption of light by material other than the substance to bemeasured (i.e., absorption of light by the support or the like). In thestructure illustrated in FIG. 36, however, the connecting portionconnecting the core layer and the support for supporting the core layeris positioned near the outer surface where the distance from the centerof the core layer is shortest in a cross-section along a planeorthogonal to the propagation direction of light through the core layer,as described below. The support therefore ends up overlapping with thearea of the outer surface where the distance from the optical axis oflight undergoing single-mode propagation is shortest. The evanescentwave extending around the core layer is greatest by the surface near theoptical axis. Hence, if the support is near this outer surface, much ofthe evanescent wave is absorbed by the material forming the support. Asensor using an optical waveguide with such a structure thus has theproblem of decreased detection sensitivity of the substance to bemeasured.

This problem with the conventional optical waveguide 10′ is nowdescribed with reference to FIG. 36. As illustrated in FIG. 36, thesupport 17′ in the optical waveguide 10′ with a conventional structureis provided between the center of the core layer 11′ and the substrate15′ within a plane (i.e. the cross-section illustrated in FIG. 36) thatis orthogonal to the longitudinal direction, i.e. the propagationdirection of light L, and that includes the support 17′. When theconnecting portion connecting the core layer 11′ and the support 17′ forsupporting the core layer 11′ is thus positioned at the widthwise centerof the core layer 11′ in a cross-section along a plane orthogonal to thepropagation direction of light L, the evanescent wave EW may be absorbedby the material forming the support 17′, and the region of the support17′ may become an obstacle that reduces the region of interactionbetween the evanescent wave EW and the substance to be measured.Consequently, the sensitivity of a sensor using the optical waveguide10′ decreases.

As illustrated in FIGS. 11, 12, 13, and 14, the optical waveguide 80according to the fourth embodiment is structured so that the core layer11 is supported by the first support 87 x and the second support 87 ywith respect to the substrate 15 while the space 13 is formed betweenthe core layer 11 and the substrate 15, like the conventional opticalwaveguide 10′. The core layer 11 has a symmetrical structure about thecenter in a cross-section perpendicular to the longitudinal direction.When the propagation of the infrared light IR through the core layer 11is single-mode, the optical axis OA of the infrared light IR propagatingthrough the core layer 11 overlaps with the center of the core layer 11.As illustrated in FIGS. 12 and 13, the first support 87 x and the secondsupport 87 y are therefore each shifted to one side from the widthwisecenter of the core layer 11 (to the right in FIG. 12 and to the left inFIG. 13). The first support 87 x and the second support 87 y can therebybe removed from the region where the evanescent wave EW is mostconcentrated. In other words, the respective connecting portions 871,872 of the first support 87 x and the second support 87 y connecting tothe core layer 11 are not at positions, near the outer surface, that arethe shortest distance from the center of the core layer 11 in across-section perpendicular to the longitudinal direction. To increasethe amount of exposure of the core layer 11, the first support 87 x andthe second support 87 y are provided discontinuously in the longitudinaldirection. Furthermore, to increase the mechanical strength of theoptical waveguide 80, the connecting portions 871, 872, connected to thecore layer 11, of the discontinuous first support 87 x and secondsupport 87 y are arranged alternately in the longitudinal direction ofthe core layer 11, i.e. the propagation direction of the infrared lightIR. In this way, the optical concentration measuring apparatus 8 thatincludes the optical waveguide 10 can have increased mechanical strengthwhile preventing a decrease, due to the presence of the first support 87x and the second support 87 y, in the detection characteristics of thesubstance to be measured MO insofar as possible.

Next, a method of manufacturing the optical waveguide 80 according tothe fourth embodiment is described using FIGS. 15 through 26, withreference to FIGS. 11, 12, and 13. FIG. 15 is a manufacturing processplan view of the optical waveguide 80. FIG. 16 is a manufacturingprocess end view of the optical waveguide 80 along the G-G line, the I-Iline, the H-H line, and the J-J line in FIG. 15. FIG. 17 is amanufacturing process plan view of the optical waveguide 80. FIG. 18 isa manufacturing process end view of the optical waveguide 80 along theG-G line, the I-I line, the H-H line, and the J-J line in FIG. 17. FIG.19 is a manufacturing process plan view of the optical waveguide 80.FIG. 20 is a manufacturing process end view of the optical waveguide 80along the G-G line and the I-I line in FIG. 19. FIG. 21 is amanufacturing process end view of the optical waveguide 80 along the H-Hline in FIG. 19. FIG. 22 is a manufacturing process end view of theoptical waveguide 80 along the J-J line in FIG. 19. FIG. 23 is amanufacturing process plan view of the optical waveguide 80. FIG. 24 isa manufacturing process end view of the optical waveguide 80 along theG-G line and the I-I line in FIG. 23. FIG. 25 is a manufacturing processend view of the optical waveguide 80 along the H-H line in FIG. 23. FIG.26 is a manufacturing process end view of the optical waveguide 80 alongthe J-J line in FIG. 23.

First, an SiO₂ film is formed on either or both of the support substrate15 a, which is formed from silicon and ultimately becomes the substrate15, and an active substrate 11 a, which is formed from silicon and fromwhich the core layer 11 is formed. The support substrate 15 a and theactive substrate 11 a are then stuck together, with the SiO₂ filmtherebetween, and thermally treated to be joined. The active substrate11 a is then ground, polished, or the like to a predetermined thicknessto adjust the film thickness of the active substrate 11 a. Consequently,as illustrated in FIGS. 15, 16, an SOI substrate 100 is formed to have a“silicon-insulating layer-silicon” structure that includes the supportsubstrate 15 a, a BOX layer 17 a formed on the support substrate 15 a,and the active substrate 11 a formed on the BOX layer 17 a.

Next, lithography and etching are used on the SOI substrate 100 to etchthe active substrate 11 a and form a core layer 11 in the shape of arectangular parallelepiped. Consequently, as illustrated in FIGS. 17,18, an optical waveguide main portion 80 a is formed to include theplate-shaped support substrate 15 a, the plate-shaped BOX layer 17 aformed on the support substrate 15 a, and the core layer 11 formed as arectangular prism on a portion of the BOX layer 17 a.

Next, as illustrated in FIGS. 19 to 21, a mask layer M3 covering aportion of the core layer 11 and the BOX layer 17 a is formed. The masklayer M3 is positioned towards one end from the widthwise center of thecore layer 11 and is arranged to alternate discontinuously. Asillustrated in FIG. 22, the core layer 11 and the BOX layer 17 a areexposed, without being covered by the mask layer M3, in a portion in thelongitudinal direction. The mask layer M3 may be a photoresist or a hardmask such as a silicon nitride film.

Next, a portion of the BOX layer 17 a of the optical waveguide mainportion 80 a is removed by wet etching or the like, with the mask layerM3 as a mask. Consequently, as illustrated in FIGS. 23 to 26, the firstsupport 87 x and the second support 87 y are formed to alternatediscontinuously at positions towards both sides (the right in FIG. 24and the left in FIG. 25) from the widthwise center of the core layer 11(i.e. positions shifted in the width direction from the optical axis OAof the infrared light propagating through the core layer 11). Thisyields a structure in which a portion of the core layer 11 is separatedfrom the substrate 15. In other words, the connecting portions 871, 872of the first support 87 x and the second support 87 y connected to thecore layer 11 are not located at positions, on the outer surface, thatare the shortest distance from the center of the core layer 11 (thewidthwise center of the outer surface when the core layer 11 iselongated widthwise as in FIGS. 24, 25) in a plane that is orthogonal tothe longitudinal direction, i.e. the propagation direction of theinfrared light, and that includes the first support 87 x or the secondsupport 87 y. Rather, the connecting portions 871, 872 are eachpositioned towards one of the ends from the widthwise center of the corelayer 11 and are formed to alternate discontinuously along thepropagation direction of the infrared light. The space 13 is formedbetween the center of the core layer 11 and a principal surface 15 s ofthe substrate 15.

Subsequently, the mask layer M3 is etched. While formation of thegrating couplers is omitted in the fourth embodiment, grating couplers118, 119 such as the ones in FIG. 11 may be formed at the same time as,or before or after, formation of the core layer 11 illustrated in FIG.17. The mask layer M3 illustrated in FIG. 19 may then be formed.Formation of the slit-shaped grating coupler 118 at one end in thelongitudinal direction of the core layer 11 and the slit-shaped gratingcoupler 119 at the other end in the longitudinal direction of the corelayer 11 yields the structure in FIG. 11.

Next, the support substrate 15 a is cut in a predetermined region toseparate the optical waveguide main portion 80 a. This completes theoptical waveguide 80 (see FIGS. 11, 12, 13, 14), in which the firstsupport 87 x and the second support 87 y alternate discontinuously atpositions shifted, in the width direction of the core layer 11, from theoptical axis OA of the infrared light propagating through the core layer11.

Furthermore, as illustrated in FIG. 11, the light source 20 is installedto be capable of emitting the infrared light IR onto the grating coupler118 of the optical waveguide 80, and the light detector 40 is disposedto be capable of detecting the infrared light IR emitted from thegrating coupler 119 of the optical waveguide 10, thereby completing theoptical concentration measuring apparatus 8.

The optical waveguide 80 thus has a structure in which the first support87 x and the second support 87 y that support the core layer 11 areshifted from a position on the outer surface overlapping the center in across-section perpendicular to the longitudinal direction of the corelayer 11 (where the distance from the optical axis OA of lightpropagating through the core layer 11 is shortest) to be locatedrespectively at each side of the widthwise center of the core layer 11.This structure can prevent a decrease, due to the first support 87 x andthe second support 87 y, in the detection characteristics of thesubstance to be measured MO while also increasing the mechanicalstrength.

In the fourth embodiment, the first support 87 x and the second support87 y are formed in different cross-sections in the directionperpendicular to the longitudinal direction of the core layer 11.Similar effects, however, are obtained when the supports are formed inthe same cross-section in the direction perpendicular to thelongitudinal direction of the core layer 11, as illustrated in FIG. 27.

As described above, the fourth embodiment can provide the opticalwaveguide 80 and the optical concentration measuring apparatus 8 thathave the first support 87 x and the second support 87 y for supportingthe core layer 11 without reducing the sensitivity of a sensor.

Furthermore, the optical waveguide 80 according to the fourth embodimentcan increase the amount of interaction between the evanescent wave EW oflight propagating through the core layer 11 and the substance to bemeasured MO and can reduce the amount of the evanescent wave EW that isabsorbed by the first support 87 x and the second support 87 y. Theoptical waveguide 80 according to the fourth embodiment can therebystably detect the substance to be measured MO, in a variety of modes ofuse, with high sensitivity.

FIFTH EMBODIMENT

An optical waveguide and an optical concentration measuring apparatusaccording to the fifth embodiment of the present disclosure aredescribed with reference to FIG. 28 through FIG. 34.

An optical waveguide 90 according to the fifth embodiment is describedas using silicon for the core material, but the core material is notlimited to silicon and may be any material that functions as an opticalwaveguide, such as GaAs.

FIG. 28 and FIG. 29 illustrate the schematic configuration of an opticalconcentration measuring apparatus 9 according to the fifth embodimentand are also conceptual drawings of the ATR method using the opticalwaveguide 90 according to the fifth embodiment.

As illustrated in FIG. 28, the optical concentration measuring apparatus9 is installed and used in an exterior space 2 containing a gas whoseconcentration or the like is to be detected. The optical concentrationmeasuring apparatus 9 includes the optical waveguide 90 according to thefifth embodiment, a light source 20 capable of causing light (infraredlight IR in the fifth embodiment) to enter a core layer 91 provided inthe optical waveguide 90, and a light detector (an example of adetector) 40 capable of detecting the infrared light IR that haspropagated through the core layer 91.

The optical waveguide 90 includes the core layer 91, through which theinfrared light IR (an example of light) can propagate in thelongitudinal direction (the left-right direction in FIG. 28), and aprotective film 14 (details provided below) formed on at least a portionof the surface of the core layer 91. The optical waveguide 90 includes asubstrate 15, a cladding layer (an example of a support) 97 formed onthe substrate 15, and the core layer 91 formed on the cladding layer 97.The cladding layer 97 joins the substrate 15 and the core layer 91. Thecore layer 91 and the substrate 15 are formed from silicon (Si), and thecladding layer 97 is formed from silicon dioxide (SiO₂).

The substrate 15 and the cladding layer 97 are plate-shaped. The corelayer 91 is a rectangular parallelepiped. The optical waveguide 90includes a grating coupler 118 formed at one end of the core layer 91 inthe longitudinal direction and a grating coupler 119 formed at the otherend of the core layer 91 in the longitudinal direction. The gratingcoupler 118 is disposed below the light source 20. The grating coupler118 couples the infrared light IR incident from the light source 20 withthe infrared light IR propagating through the core layer 91. The gratingcoupler 119 is disposed below the light detector 40. The grating coupler119 extracts the infrared light IR propagating through the core layer 91and emits the infrared light IR towards the light detector 40.

FIG. 29 is a cross-section of the optical concentration measuringapparatus along the K-K line in FIG. 28. As illustrated in FIG. 29, theoptical waveguide 90 according to the fifth embodiment includes a corelayer 91, through which light (infrared light in the fifth embodiment)propagates in the longitudinal direction, and the protective film 14formed between the core layer 91 and a substance to be detected presentin the exterior space 2. In a cross-section of at least a portionperpendicular to the longitudinal direction of the core layer (across-section along the K-K line, i.e. the cross-section in FIG. 29),the entire surface of the core layer is not exposed. In the fifthembodiment, the core layer 91 is formed from silicon. The protectivefilm 14 is provided to suppress natural oxidation of the surface of thecore layer 91. The protective film 14 is preferably provided in such away that the surface of the core layer 91 is not exposed. The protectivefilm 14 may therefore be formed from any material that can suppressnatural oxidation of the surface of the core layer 91. When, forexample, the core layer 91 is formed from silicon, the protective film14 may be formed as a silicon nitride film, a silicon oxynitride film,or a laminate of a silicon oxide film and a silicon nitride film. A filmincluding nitrogen has the effect of suppressing oxidation of thesurface of the core layer 91. As the nitrogen content is higher, theeffect of suppressing oxidation increases. The protective film 14 may bea film that includes nitrogen and has a nitrogen content of 1% or morein at least a partial region of the film. A silicon nitride film and asilicon oxynitride film have a high refractive index difference withrespect to silicon and are therefore excellent materials for forming acladding layer. Furthermore, a silicon nitride film and a siliconoxynitride film exhibit little absorption of infrared light and cantherefore minimize a reduction in the detection sensitivity of thematerial to be measured due to formation of the protective film 14 onthe surface of the core layer 91.

The protective film 14 is preferably thin, as long as the thickness issufficient to suppress natural oxidation of the core layer 91. Thereason is that the region of interaction between the evanescent wave andthe substance to be measured can be expanded as the protective film 14is thinner. If the protective film 14 is formed to be too thick, thenalthough degradation in the characteristics of the optical waveguide 90can be prevented, the amount of interaction between the evanescent waveand the substance to be measured decreases. This decreases thesensitivity that the optical concentration measuring apparatus 9 isintended to have as a detection apparatus. Accordingly, the thickness ofthe protective film 14 may be 1 nm or more and less than 20 nm, or maybe 2 nm or more and less than 5 nm. The lower limit of 1 nm on thethickness of the protective film 14 represents the thickness roughlynecessary for stopping growth of a natural oxide film on the surface ofthe core layer 91. The upper limit of 20 nm on the thickness of theprotective film 14 takes into consideration the change, depending on thematerial for forming the protective film 14 and the formation method, inthe effect of suppressing natural oxide film growth.

Next, operations of the optical waveguide 90 and the opticalconcentration measuring apparatus 9 are described with reference to FIG.28. The optical concentration measuring apparatus 9 detects a gas in theexterior space 2 using the ATR method. In the ATR method, infrared lightis guided into an optical waveguide from one grating coupler, ispropagated through the optical waveguide, and is extracted at the sideof another grating coupler. The amount of the infrared light is thendetected by a light detector located ahead.

In greater detail, infrared light IR emitted from the light source 20 isincident on the grating coupler 118 provided in the optical waveguide190, as illustrated in FIG. 28. The incident infrared light IR isdiffracted and guided into the core layer 91 at a predetermined angle bythe grating coupler 118 to be coupled with the infrared light IRpropagating through the core layer 91.

The infrared light IR guided through the core layer 91 is repeatedlyreflected at the boundary between the core layer 91 and the protectivefilm 14, and at the boundary between the core layer 91 and the claddinglayer 97, and arrives at the grating coupler 119. An evanescent wave EWis produced in the exterior space 2 via the protective film 14 when theinfrared light IR is reflected at the boundary between the core layer 91and the protective film 14. The amount of the evanescent wave EWextending into the exterior space 2 (the extension depth) differs inaccordance with the substance to be detected present in the exteriorspace 2, and the amount of the evanescent wave EW that is absorbed alsodiffers in accordance with the substance to be detected present in theexterior space 2. Therefore, the intensity of the infrared light IR thatpropagates through the core layer 91 and arrives at the grating coupler119 differs in accordance with the substance to be detected present inthe exterior space 2.

The grating coupler 119 diffracts the arriving infrared light IR andextracts the infrared light IR into the exterior space 2 towards thelight detector 40. The concentration or the like of the substance to bedetected present in the exterior space 2 can be detected by analyzingthe intensity of the infrared light IR detected by the light detector40.

Next, a method of manufacturing the optical waveguide 90 is describedusing FIGS. 30 and 31, with reference to FIGS. 28 and 29. FIG. 30 is amanufacturing process end view of the optical waveguide 90 along the K-Kline in FIG. 1.

First, an SiO₂ film is formed on either or both of the support substrate15 a, which is formed from silicon and ultimately becomes the substrate15, and an active substrate 91 a, which is formed from silicon and fromwhich the core layer 11 is formed. The support substrate 15 a and theactive substrate 11 a are then stuck together, with the SiO₂ filmtherebetween, and thermally treated to be joined. The active substrate11 a is then ground, polished, or the like to a predetermined thicknessto adjust the film thickness of the active substrate 11 a. Consequently,as illustrated in FIG. 30, an SOI substrate 100 is formed to have a“silicon-insulating layer-silicon” structure that includes the supportsubstrate 15 a, a BOX layer 17 a formed on the support substrate 15 a,and the active substrate 11 a formed on the BOX layer 17 a.

Next, lithography and etching are used on the SOI substrate 100 to etchthe active substrate 11 a and form a core layer 91 in the shape of arectangular parallelepiped. Consequently, as illustrated in FIG. 31, anoptical waveguide main portion 90 a is formed to include theplate-shaped substrate 15, the plate-shaped cladding layer 97 formed onthe substrate 15, and the core layer 91 formed as a rectangular prism ona portion of the cladding layer 97.

Subsequently, a nitride film is deposited using thermal CVD, oroxidation is performed under an atmosphere including NO or N₂O, to forma film including nitrogen on the surface of the core layer 91. Thisnitride film is formed to a thickness of 1 nm or more and less than 20nm, for example. In this way, the protective film 14 is formed on thethree sides of the core layer 91 other than the side in contact with thecladding layer 97, as illustrated in FIG. 29.

When using deposition by thermal CVD in the method of forming a filmincluding nitrogen, a film including nitrogen is also formed on thesurface of the cladding layer 97. On the other hand, when usingoxidation under an atmosphere including NO or N₂O, a film includingnitrogen is not formed on the surface of the cladding layer 97. Amongthese formation methods, oxidation under an atmosphere including NO orN₂O is adopted in the fifth embodiment to form the protective film 14including nitrogen on the surface of the core layer 91.

As an example, FIG. 32 is a graph illustrating the nitrogenconcentration when the silicon surface is oxidized under an atmosphereincluding NO to form an oxynitride film with a thickness of 3 nm. Thehorizontal axis in the graph in FIG. 32 represents depth (nm) from thesurface of the oxynitride film, and the vertical axis of the graphrepresents the nitrogen concentration (%) in the oxynitride film and thesilicon. The distribution in FIG. 32 indicates a nitrogen concentrationof several percent near the boundary between the silicon and theoxynitride film. The nitrogen concentration in the oxynitride film canbe adjusted by changing the NO gas flow rate during oxidation.

Furthermore, the slit-shaped grating coupler 118 is subsequently formedat one end in the longitudinal direction of the core layer 91, and theslit-shaped grating coupler 119 is formed at the other end in thelongitudinal direction of the core layer 91. As illustrated in FIG. 28,this completes the optical waveguide 90 having the protective film 14,which includes nitrogen and has a thickness of 1 nm or more and lessthan 20 nm, on the surface of the core layer 91. The order of theprocess to form the grating couplers and the process to form the filmincluding nitrogen on the surface of the core layer 91 may be reversed.

Furthermore, while omitted from the drawings, the light source 20 isinstalled to be capable of emitting the infrared light IR onto thegrating coupler 118 of the optical waveguide 90, and the light detector40 is disposed to be capable of detecting the infrared light IR emittedfrom the grating coupler 119 of the optical waveguide 90, therebycompleting the optical concentration measuring apparatus 9.

Next, another example configuration of the optical waveguide 90 and theoptical concentration measuring apparatus 9 is described using FIGS. 33and 34. FIGS. 33 and 34 illustrate the optical waveguide 90 and theoptical concentration measuring apparatus 9 according to another exampleconfiguration. FIG. 34 is a cross-section of the optical concentrationmeasuring apparatus 9 along the L-L line in FIG. 33.

As illustrated in FIG. 33, the optical waveguide 90 according to theother example configuration has a floating core layer 91 in whichsupports 98 for supporting the core layer 91 are only present in certainportions. Specifically, the core layer 91 is supported by a plurality ofcolumnar supports 98 formed at predetermined intervals on the substrate15.

When forming the optical waveguide 90 to include the floating core layer91, an additional process is performed after the core layer 91 is formedas a rectangular prism on the BOX layer 17 a in FIG. 30 (i.e. in a statecorresponding to FIG. 31). Specifically, lithography, wet etching, orthe like is used to partially remove the BOX layer 17 a except in aregion where the core layer 91 is to be supported by the supports 98. Aplurality of supports 98 are thus formed in portions between the corelayer 91 and the substrate 15. In this way, the core layer 91 can befloated by the supports 98, yielding a space in portions between thecore layer 91 and the substrate 15. In the optical waveguide 90according to the other example configuration, the air in the spacebetween the core layer 91 and the substrate 15 functions as a claddinglayer. The optical waveguide 90 according to the other exampleconfiguration may also have a structure such that adjacent supports 98are connected by the BOX layer 17 a remaining on the substrate 15 afteretching, as illustrated in FIGS. 33 and 34.

After formation of the supports 98, a nitride film is deposited usingthermal CVD, or oxidation is performed under an atmosphere including NOor N₂O, to form a film including nitrogen on the surface of the corelayer 91. The protective film 14 can thus be formed around the entiresurface of the core layer 91 in a cross-section perpendicular to thelongitudinal direction of the core layer 91, as illustrated in FIG. 34,and deterioration of the core layer 91 can effectively be suppressed.The optical waveguide 90 and optical concentration measuring apparatus 9including the floating core layer 91 illustrated in FIG. 33 arecompleted by a manufacturing process similar to the one described abovebeing performed after formation of the protective film 14. A detailedexplanation is omitted.

Here, the effects of the optical waveguide 90 and the opticalconcentration measuring apparatus 9 according to the fifth embodiment(including the optical waveguide 90 and the optical concentrationmeasuring apparatus 9 according to the other configuration example) aredescribed. In a sensor using the ATR method, greater exposure of thesurface of the core layer forming the optical waveguide increases theamount of the evanescent wave that interacts with the substance to bedetected. This improves the sensitivity of the sensor. If the core layeris exposed, however, the surface state of the core layer changesunexpectedly due to the external environment, and the sensor performanceends up changing over time. Natural oxidation is one example of changein the core layer due to the external environment. As illustrated inFIG. 35, the evanescent wave is greatest at the boundary between astructure 51 (corresponding to the core layer in the case of an opticalwaveguide) and an external substance 53. The intensity E2 of theevanescent wave decreases as the distance from this boundary increases.The sensitivity of the sensor is therefore more easily affected as thestate changes in a region closer to the boundary between the structure51 and the substance 53. Accordingly, the effect of the natural oxidefilm occurring on the outermost surface of the structure 51 isdisastrous for a sensor required to have high sensitivity.

By contrast, the optical waveguide 90 includes the protective film 14 onthe surface of the core layer 91. The optical waveguide 90 is thereforeconfigured so that the core layer 91 does not come into direct contactwith the exterior space 2. This allows the surface state of the corelayer 91 to be maintained in the initial state, without changing overtime. Consequently, the optical waveguide 90 and the opticalconcentration measuring apparatus 9 can prevent age-related degradation.Furthermore, the protective film 14 is formed as a silicon nitride filmor silicon oxynitride film that absorbs little infrared light and has athickness of 1 nm or more and less than 20 nm. The optical waveguide 90can thus prevent a reduction in the amount of interaction between theevanescent wave and the substance to be detected. Consequently, theoptical waveguide 90 and the optical concentration measuring apparatus 9can detect the substance to be detected with high sensitivity.

As described above, the optical waveguide and optical concentrationmeasuring apparatus according to the fifth embodiment (including theoptical waveguide and the optical concentration measuring apparatusaccording to the other configuration example) include the protectivefilm on the surface of the core layer and can therefore preventdegradation of the surface state of the core layer due to the externalenvironment. Consequently, the optical waveguide and the opticalconcentration measuring apparatus according to the fifth embodiment candetect the substance to be detected with high sensitivity and canprevent age-related degradation.

Embodiments of the present disclosure have been described, but thetechnical range of the present disclosure is not limited to thetechnical range of the above embodiments. A variety of modifications orimprovements may be made to the above embodiments, and it is clear fromthe scope of the patent claims that embodiments with such modificationsor improvements can be included within the technical range of thepresent disclosure.

REFERENCE SIGNS LIST

1, 8, 9 Optical concentration measuring apparatus

2 Exterior space

10, 60, 70, 80, 90, 10′ Optical waveguide

10 a, 70 a, 80 a, 90 a Optical waveguide main portion

11, 91, 11′ Core layer

11 a Active substrate

13, 13 a, 13 b, 13 c Space

14 Protective film

15, 15′ Substrate

15 a Support substrate

15 s Principal surface

17, 17 x, 17 y, 97, 98, 17′ Support (cladding layer)

87 x, 87 y First support, second support

17 a BOX layer

20 Light source

40 Light detector

51 Structure

53 Substance

100 SOI substrate

111, 112, 113 Separated portion

118, 119 Grating coupler

171, 172, 173 Connecting portion

871, 872 Connecting portion of first support, connecting portion ofsecond support

a1 Axis where optical axis OA1 will be formed

a2 Axis where optical axis OA2 will be formed

a3 Axis where optical axis OA3 will be formed

Cm Center line of mask layer

OA, OA1, OA2, OA3 Optical axis

EW Evanescent wave

IR Infrared light

L Light

MO Substance to be measured

M1, M2, M3 Mask layer

NP Position having shortest distance from the center to the outersurface

The invention claimed is:
 1. An optical waveguide for use in an opticalconcentration measuring apparatus for measuring concentration of a gasto be measured or a liquid to be measured, the optical waveguidecomprising: a substrate; a core layer that extends along a longitudinaldirection and through which light can propagate; and a support formedfrom a material with a smaller refractive index than the core layer andconfigured to connect at least a portion of the substrate and at least aportion of the core layer to support the core layer with respect to thesubstrate; wherein a connecting portion of the support connected to thecore layer is shifted from a position having a shortest distance from acenter to an outer surface in a cross-section perpendicular to thelongitudinal direction of the core layer.
 2. The optical waveguide ofclaim 1, wherein at least a portion of the core layer is provided in amanner allowing contact with the gas to be measured or the liquid to bemeasured.
 3. An optical waveguide comprising: a substrate; a core layerthat extends along a longitudinal direction and through which light canpropagate; and a support formed from a material with a smallerrefractive index than the core layer and configured to connect at leasta portion of the substrate and at least a portion of the core layer tosupport the core layer with respect to the substrate; wherein at least aportion of the core layer is provided in a manner allowing contact witha gas or a liquid; and wherein a connecting portion of the supportconnected to the core layer is shifted from a position having a shortestdistance from a center to an outer surface in a cross-sectionperpendicular to the longitudinal direction of the core layer.
 4. Theoptical waveguide of claim 1, wherein the support comprises a pluralityof the connecting portions separated spatially.
 5. The optical waveguideof claim 1, wherein at least a portion of the core layer is floatingwithout being joined to the support.
 6. The optical waveguide of claim1, wherein the core layer comprises a protective film formed on at leasta portion of a surface of the core layer and having a thickness of 1 nmor more and less than 20 nm.
 7. The optical waveguide of claim 6,wherein the protective film is a silicon nitride film or a siliconoxynitride film.
 8. The optical waveguide of claim 1, wherein lightpropagating through the core layer is infrared light serving as ananalog signal.
 9. The optical waveguide of claim 1, wherein the supportcomprises a first support and a second support; and wherein in a widthdirection of the core layer, a connecting portion of the first supportis positioned between a center and one end, and a connecting portion ofthe second support is positioned between the center and the other end.10. The optical waveguide of claim 9, wherein the connecting portion ofthe first support and the connecting portion of the second support areintermittently present along the longitudinal direction.
 11. The opticalwaveguide of claim 9, wherein the connecting portion of the firstsupport and the connecting portion of the second support are alternatelypresent along the longitudinal direction.
 12. The optical waveguide ofclaim 9, wherein at least a portion of the core layer is exposed or iscovered by a thin film.
 13. The optical waveguide of claim 9, whereinthe first support and the second support are not present in an entireregion between the core layer and the substrate in a cross-sectionperpendicular to the longitudinal direction at a position of at least aportion in the longitudinal direction of the core layer.
 14. The opticalwaveguide of claim 9, wherein the connecting portion of the firstsupport and the second support is shaped to expand in the longitudinaldirection of the core layer with proximity to the center of the corelayer from an edge of the core layer in the width direction of the corelayer.
 15. An optical concentration measuring apparatus comprising: theoptical waveguide of claim 1; a light source capable of causing light toenter the core layer; and a detector capable of detecting light that haspropagated through the core layer.
 16. The optical concentrationmeasuring apparatus of claim 15, wherein the light source causesinfrared light having a wavelength of 2 μm or more and less than 10 μmto enter the core layer.